(a) Field of the Invention
This invention relates to a fuel supply device for carburetors and more particularly to a fuel supply device wherein an air-fuel ratio of a mixture to be supplied to an engine is kept proper by controlling the fuel flow rate so that the height of the fuel column to be formed within a negative pressure passageway for detecting the sucked air flow rate communicating with a fuel passageway may be kept at a predetermined value.
(b) Description of the prior art
A conventional fuel supply device of this kind is mentioned in European patent publication No. 0207796 filed previously by the applicant of the present application. In this device, a solenoid valve is arranged in the course of a fuel passageway opening at one end to a variable venturi part of a suction bore and communicating at the other end with a fuel supply source through a jet, a liquid level sensor is arranged in the course of a negative pressure passageway opening at one end to a stationary venturi part generating a negative pressure lower than the negative pressure generated by the variable venturi on the upstream side of the variable venturi and communicating at the other end with the fuel passageway between the above-mentioned jet and the solenoid valve and a control circuit controlling the opening and closing of the solenoid valve by receiving a signal from the liquid level sensor so that the height of the fuel column to be formed within a negative pressure passageway may be fixed is provided.
Now, there has been a problem that, in the case of the above-mentioned conventional device, when the amount of air to be sucked increases in a high load range and the air passing area, that is, the valve opening degree becomes large, the pressure difference between the variable venturi and the stationary venturi will become so small that, even if the solenoid valve is fully opened, the fuel column within the negative pressure passageway will not be able to be lowered, the fuel will be delivered into the suction bore from the negative pressure passageway and the air-fuel ratio of the mixture will be no longer able to be controlled. If the valve opening degree in the variable venturi is limited, this problem will be solved but the flow resistance within the suction bore will become so large as to be undesirable.